1. Field of the Invention
The present invention relates generally to the field of integrated circuit manufacturing technology and, more particularly, to an improved method for depositing thin films.
2. Background of the Related Art
In the manufacturing of integrated circuits, numerous microelectronic circuits are simultaneously manufactured on semiconductor substrates. These substrates are usually referred to as wafers. A typical wafer is comprised of a number of different regions, known as die regions. When fabrication is complete, the wafer is cut along these die regions to form individual die. Each die contains at least one microelectronic circuit, which is typically replicated on each die. One example of a microelectronic circuit which can be fabricated in this way is a dynamic random access memory (“DRAM”).
Although referred to as semiconductor devices, integrated circuits are in fact fabricated from numerous materials of varying electrical properties. These materials include insulators or dielectrics, such as silicon dioxide, and conductors, such as aluminum or tungsten, in addition to semiconductors, such as silicon and germanium.
In the manufacture of integrated circuits, conductive paths are formed to connect different circuit elements that have been fabricated within a die. One method to make these connections is through the use of openings in intermediate insulative layers. These openings are typically referred to as “contact openings” or “vias.” A contact opening is typically created to expose an active region, commonly referred to as a doped region, while vias traditionally refer to any conductive path between any two or more layers in a semiconductor device.
After a contact opening, for instance, has been formed to expose an active region of the semiconductor substrate, an enhanced doping may be performed through the opening to create a localized region of increased carrier density within the bulk substrate. This enhanced region provides a better electrical connection with the conductive material which is subsequently deposited within the opening. One method of increasing conductivity further involves the deposition of a thin titanium-containing film, such as titanium silicide, over the wafer so that it covers the enhanced region prior to deposition of the conductive layer. Thin films of titanium-containing compounds also find other uses as well in the fabrication of integrated circuits. For example, titanium nitride is used as a diffusion barrier to prevent chemical attack of the substrate, as well as to provide a good adhesive surface for the subsequent deposition of tungsten.
Indeed, many reasons exist for depositing thin films between adjacent layers in a semiconductor device. For example, thin films may be used to prevent interdiffusion between adjacent layers or to increase adhesion between adjacent layers. Titanium nitride, titanium silicide, and metallic titanium are known in the art as materials that can be deposited as thin films to facilitate adhesion and to reduce interdiffusion between the layers of a semiconductor device. Other films that may be useful for these purposes also include titanium tungsten, tantalum nitride, and the ternary alloy composed of titanium, aluminum, and nitrogen.
The deposition of titanium-containing films is just one example of a step in the manufacture of semiconductor wafers. Indeed, any number of thin films, insulators, semiconductors, and conductors may be deposited onto a wafer to fabricate an integrated circuit. As the size of the microelectronic circuits, and therefore the size of die regions, decreases, the percentage of reliable circuits produced on any one wafer becomes highly dependent on the ability to deposit these thin films uniformly across the surface of the wafer. This includes uniform deposition on horizontal surfaces, slanted surfaces, and vertical surfaces, including those surfaces which define the walls and base of contacts and vias. If these thin films are not deposited in a uniform manner, gaps may be created which prevent the thin film from fully performing its function. The likelihood of the existence of these gaps tends to increase as the films become thinner.
Films may be deposited by several different methods, such as thermal growth, sputter deposition, spin-on deposition, chemical vapor deposition (CVD), and plasma enhanced chemical vapor deposition (PECVD). In thermal growth, the wafer substrate is heated at precisely controlled temperatures, typically between 800 and 1200° C., with a choice of ambient gases. The high temperature promotes the reaction between the ambient gas and the wafer substrate. For instance, films of silicon dioxide are often produced by this method. The problem with this method is the extremely high deposition temperatures required. Extremely high temperatures are a concern for two reasons. First, high temperature may be incompatible with or even detrimental to other elements of the integrated circuit, and, second, excessive cycling from low to high temperatures can damage a circuit, thereby reducing the percentage of reliable circuits produced from a wafer. Therefore, a lower deposition temperature is typically preferred as long as the characteristics of the deposited film are unaffected.
In sputter deposition, the material to be deposited is bombarded with positive inert ions. Once the material exceeds its heat of sublimation, atoms are ejected into the gas phase where they are subsequently deposited onto the substrate, which may or may not be negatively biased. Sputter deposition has been widely used in integrated circuit processes to deposit titanium-containing films. The primary disadvantage of sputter deposition is that it results in films having poor step coverage, so it may not be widely useable in submicron processes. Films deposited by sputter deposition on slanted or vertical surfaces do not exhibit uniform thickness, and the density of films deposited on these surfaces is usually not as high as the films deposited on horizontal surfaces.
In spin-on deposition, the material to be deposited is mixed with a suitable solvent and spun onto the substrate. The primary disadvantage of spin-on deposition is that nominal uniformity can only be achieved at relatively high thicknesses. Therefore, this method is primarily used for the deposition of photoresist and the like. It is generally not useful for the deposition of thin films.
As previously indicated, the trend for reducing the size of die regions has dictated the reduction of the thickness of many deposited films. These thin films need to have improved step coverage to reduce the number of gaps in the films and to increase the yield of operable devices. Of the methods discussed above, CVD and PECVD are best suited to deposit the thinnest films, as films deposited by sputter deposition on slanted or vertical surfaces do not exhibit the degree of uniformity obtainable by CVD and PECVD.
In CVD, the gas phase reduction of highly reactive chemicals under low pressure results in very uniform thin films. A basic CVD process used for depositing titanium involves a given composition of reactant gases and a diluent which are injected into a reactor containing one or more silicon wafers. The reactor is maintained at selected pressures and temperatures sufficient to initiate a reaction between the reactant gases. The reaction results in the deposition of a thin film on the wafer. If the gases include hydrogen and a titanium precursor, a titanium-containing film will be deposited. For example, if, in addition to hydrogen and the titanium precursor, the reactor contains a sufficient quantity of nitrogen or a silane, the resulting titanium-containing film will be titanium nitride and titanium silicide respectively. Plasma enhanced CVD is a form of CVD that includes bombarding the material to be deposited with a plasma to generate chemically reactive species at relatively low temperatures.
Chemical vapor deposition is typically carried out in one of two types of reactor. One type of reactor is called a hot wall reactor. A hot wall reactor is operated at a low pressure, typically 1 Torr or less, and high temperatures, typically 600° C. or greater. The other type of reactor is called a cold wall reactor. A cold wall reactor is operated at atmospheric pressure and low temperatures, typically 400 to 600° C.
The primary advantage of the hot wall reactor is that deposited films exhibit excellent purity and uniform step coverage. However, the hot wall reactor process is also characterized by low deposition rates, high temperatures, and the potential for the occurrence of unwanted reactions on the walls of the reaction chamber. Conversely, the cold wall reactor exhibits high deposition rates but poor step coverage.
Exposure to extreme temperatures and excessive cycling from low to high temperatures during the fabrication of integrated circuits can render the circuits useless. Therefore, a process for depositing films exhibiting uniform step coverage that can be conducted with a minimum of exposure to elevated temperatures could have a dramatic impact on the yield of reliable circuits. It has been thought that PECVD is the best method of achieving this result. In fact, plasma deposition has been used to produce titanium-containing films in a cold wall reactor maintained at approximately 400° C. The result of this deposition is thin titanium-containing films exhibiting good step coverage and growth rate.
However, the current plasma deposition technology does have its limitations. Because of the higher pressures associated with deposition in a cold wall reactor, it is difficult to deposit films that exhibit a high degree of uniform coverage in contacts and vias having high aspect ratios. This difficulty extends to both the vertical surfaces of the contacts and vias as well as the horizontal surfaces at the base of the contacts and vias.
The present invention may address one or more of the problems set forth above.